Electric organ invented in 1934 as a cheaper alternative to pipe organs for churches, later becoming popular for jazz, blues and rock in the 1960s and 70s. Drawbars above the manuals (rows of keys) emulate the "stops" of pipe organs, by enabling/disabling different overtones (such as fundamental, octave below, octave above, fifth above, etc.), and are labeled similarly to pipe organ stops. The octave of reversed-color keys on each manual (black naturals and white sharps/flats) are for drawbar presets and do not actually play notes.
Hammond organs are generally played through a Leslie speaker, which rotates internally, producing a tremolo effect.